text: // Copyright 2019 DeepMind Technologies Ltd. All rights reserved.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  #ifndef THIRD_PARTY_OPEN_SPIEL_GAMES_CONNECT_FOUR_H_
  #define THIRD_PARTY_OPEN_SPIEL_GAMES_CONNECT_FOUR_H_
  
  #include <array>
  #include <map>
  #include <memory>
  #include <string>
  #include <vector>
  
  #include "open_spiel/spiel.h"
  
  // Simple game of Connect Four
  // https://en.wikipedia.org/wiki/Connect_Four
  //
  // Minimax values (win/loss/draw) available for first 8 moves, here:
  // https://archive.ics.uci.edu/ml/datasets/Connect-4
  //
  // Parameters: none
  
  namespace open_spiel {
  namespace connect_four {
  
  // Constants.
  inline constexpr int kNumPlayers = 2;
  inline constexpr int kRows = 6;
  inline constexpr int kCols = 7;
  inline constexpr int kNumCells = kRows * kCols;
  inline constexpr int kCellStates =
      1 + kNumPlayers;  // player 0, player 1, empty
  
  // Outcome of the game.
  enum class Outcome {
    kPlayer1 = 0,
    kPlayer2 = 1,
    kUnknown,
    kDraw,
  };
  
  // State of a cell.
  enum class CellState {
    kEmpty,
    kNought,
    kCross,
  };
  
  // State of an in-play game.
  class ConnectFourState : public State {
   public:
    ConnectFourState(std::shared_ptr<const Game>);
    explicit ConnectFourState(std::shared_ptr<const Game> game,
                              const std::string& str);
    ConnectFourState(const ConnectFourState& other) = default;
  
    Player CurrentPlayer() const override;
    std::vector<Action> LegalActions() const override;
    std::string ActionToString(Player player, Action action_id) const override;
    std::string ToString() const override;
    bool IsTerminal() const override;
    std::vector<double> Returns() const override;
    std::string InformationStateString(Player player) const override;
    std::string ObservationString(Player player) const override;
    void ObservationTensor(Player player,
                           std::vector<double>* values) const override;
    std::unique_ptr<State> Clone() const override;
    std::string Serialize() const override;
  
   protected:
    void DoApplyAction(Action move) override;
  
   private:
    CellState& CellAt(int row, int col);
    CellState CellAt(int row, int col) const;
    bool HasLine(Player player) const;  // Does this player have a line?
    bool HasLineFrom(Player player, int row, int col) const;
    bool HasLineFromInDirection(Player player, int row, int col, int drow,
                                int dcol) const;
    bool IsFull() const;         // Is the board full?
    Player current_player_ = 0;  // Player zero goes first
    Outcome outcome_ = Outcome::kUnknown;
    std::array<CellState, kNumCells> board_;
  };
  
  // Game object.
  class ConnectFourGame : public Game {
   public:
    explicit ConnectFourGame(const GameParameters& params);
    int NumDistinctActions() const override { return kCols; }
    std::unique_ptr<State> NewInitialState() const override {
      return std::unique_ptr<State>(new ConnectFourState(shared_from_this()));
    }
    int NumPlayers() const override { return kNumPlayers; }
    double MinUtility() const override { return -1; }
    double UtilitySum() const override { return 0; }
    double MaxUtility() const override { return 1; }
    std::shared_ptr<const Game> Clone() const override {
      return std::shared_ptr<const Game>(new ConnectFourGame(*this));
    }
    std::vector<int> ObservationTensorShape() const override {
      return {kCellStates, kRows, kCols};
    }
    int MaxGameLength() const override { return kNumCells; }
    std::unique_ptr<State> DeserializeState(
        const std::string& str) const override;
  };
  
  inline std::ostream& operator<<(std::ostream& stream, const CellState& state) {
    switch (state) {
      case CellState::kEmpty:
        return stream << "Empty";
      case CellState::kNought:
        return stream << "O";
      case CellState::kCross:
        return stream << "X";
      default:
        SpielFatalError("Unknown cell state");
    }
  }
  
  }  // namespace connect_four
  }  // namespace open_spiel
  
  #endif  // THIRD_PARTY_OPEN_SPIEL_GAMES_CONNECT_FOUR_H_
  